The CMN interconnect is a rectangular grid comprised of "crosspoints" (XPs). Nodes of various types are attached to crosspoints. Some systems may have more than one CMN interconnect - for instance a multi-socket system would have at least one per socket.
Important CMN node types include:
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requesters (RN-F): CPUs are attached to these
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memory home nodes (HN-F): these handle all memory requests from the CPU and also contain slices of system cache
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subordinate nodes (SN-F): these interface to memory controllers, which manage DDR modules
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chip-to-chip gateways (CCG): these act as bidirectional interfaces to other CMN interconnects.
Full details of CMN can be found in Arm's product documentation.